Saturday, April 9, 2011

How it's done volume 1: Getting and optimizing for force

Check out wikipedia for the basics... You read it? Good. Now that you understand that, let's talk about engineering this gun. Our objective is to get it to fire a fast moving projectile. Chemical guns get a good velocity. It should work like that. But with magnetics. And not being so damn loud.

When considering a dynamic system (aka: shit moving), it's often helpful to consider the static case (nothing is moving). In our static case, we have a projectile near an electromagnet. During this moment, the electromagnet is exerting a force on the projectile. If the force is allowed to move the projectile some small distance, the projectile will pick up Energy = Force * Distance from it. (You may ask where the energy comes from in this case. If we observed the coil during this time, the current through it would have decreased. As the now magnetized projectile moves closer it causes current in the opposite direction of the existing current.)

So how do we build this machine to get the maximum force? After all, it looks like getting the most force is what we need to get the most energy in our projectile. If this where a chemical gun, that force could be computed as the pressure on the back of the bullet. The pressure * the area of the back of the bullet * the length of the barrel will give you the expected energy of the bullet. In that case, we could just have to add more powder to increase the pressure. We could also increase the area of the back of the bullet. This is common in many very high speed guns.

Shittily, knowing the force we're imparting is ridiculously complex with a magnetic weapon. For starters, the magnetic field strength is based on the current in the coils which is both dynamic and changes with the movement of the projectile. Secondly, the force is difficult (impossible?) to compute just from the construction of the coils and especially hard as the projectile is moving through them.

So if don't know how to find the force, we should at least be able to aim for getting more of it. I recommend downloading this Finite Element Method Magnetics (FEMM) magnetic simulator to get a sense of how the magnetic fields will work. once you get the hang of it, it'll also tell you what sort of magnetic field strength to expect given your setup. In particular, you should notice that it's hard to increase the magnetic field strength beyond 2T (or rather it doesn't increase as fast after that). This is called magnetic saturation. From what I understand, that's essentially when all the iron dipoles are already pointing in the same direction (and thus there are no more to give a boost to the field being applied to them).

If you don't download that and use it, I'll give you a hint: There's a stronger field when there's iron around the coil as well. So the first thing we now know is that to make the strongest accelerator, we need to have magnetic shielding around our coils.

And that is the point of what we've learned with this post: If you want to get more power, put magnetic shielding around your coils.